The Massive Wolf-Rayet Binary SMC WR7

We present a study of optical spectra of the Wolf--Rayet star AzV 336a (= SMC WR7) in the Small Magellanic Cloud. Our study is based on data obtained at several Observatories between 1988 and 2001. We find SMC WR7 to be a double lined WN+O6 spectroscopic binary with an orbital period of 19.56 days. The radial velocities of the He absorption lines of the O6 component and the strong He{\sc ii} emission at $\lambda$4686\AA of the WN component describe antiphased orbital motions. However, they show a small phase shift of $\sim$ 1 day. We discuss possible explanations for this phase shift. The amplitude of the radial velocity variations of He {\sc ii} emission is twice that of the absorption lines. The binary components have fairly high minimum masses, $\sim$ 18 \modot and 34 \modot for the WN and O6 components, respectively.Comment: Accepted by MNRA

Massive runaway stars in the Small Magellanic Cloud

Using archival Spitzer Space Telescope data, we identified for the first time a dozen runaway OB stars in the Small Magellanic Cloud (SMC) through the detection of their bow shocks. The geometry of detected bow shocks allows us to infer the direction of motion of the associated stars and to determine their possible parent clusters and associations. One of the identified runaway stars, AzV 471, was already known as a high-velocity star on the basis of its high peculiar radial velocity, which is offset by ~40 km/s from the local systemic velocity. We discuss implications of our findings for the problem of the origin of field OB stars. Several of the bow shock-producing stars are found in the confines of associations, suggesting that these may be "alien" stars contributing to the age spread observed for some young stellar systems. We also report the discovery of a kidney-shaped nebula attached to the early WN-type star SMC-WR3 (AzV 60a). We interpreted this nebula as an interstellar structure created owing to the interaction between the stellar wind and the ambient interstellar medium.Comment: Accepted by A&

The impact of mass-loss on the evolution and pre-supernova properties of red supergiants

The post main-sequence evolution of massive stars is very sensitive to many parameters of the stellar models. Key parameters are the mixing processes, the metallicity, the mass-loss rate and the effect of a close companion. We study how the red supergiant lifetimes, the tracks in the Hertzsprung-Russel diagram (HRD), the positions in this diagram of the pre-supernova progenitor as well as the structure of the stars at that time change for various mass-loss rates during the red supergiant phase (RSG), and for two different initial rotation velocities. The surface abundances of RSGs are much more sensitive to rotation than to the mass-loss rates during that phase. A change of the RSG mass-loss rate has a strong impact on the RSG lifetimes and therefore on the luminosity function of RSGs. At solar metallicity, the enhanced mass-loss rate models do produce significant changes on the populations of blue, yellow and red supergiants. When extended blue loops or blue ward excursions are produced by enhanced mass-loss, the models predict that a majority of blue (yellow) supergiants are post RSG objects. These post RSG stars are predicted to show much smaller surface rotational velocities than similar blue supergiants on their first crossing of the HR gap. The position in the HRD of the end point of the evolution depends on the mass of the hydrogen envelope. More precisely, whenever, at the pre-supernova stage, the H-rich envelope contains more than about 5\% of the initial mass, the star is a red supergiant, and whenever the H-rich envelope contains less than 1\% of the total mass the star is a blue supergiant. For intermediate situations, intermediate colors/effective temperatures are obtained. Yellow progenitors for core collapse supernovae can be explained by the enhanced mass-loss rate models, while the red progenitors are better fitted by the standard mass-loss rate models.Comment: 19 pages, 11 figures, 6 tables, accepted for publication in Astronomy and Astrophysic

Variable Star Candidates in an ACS Field of M31

A search for variable stars is performed using two epochs of Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging data for a 9.28 square arcminute portion of M31. This data set reveals 254 sources that vary by at least 4-sigma between epochs. The positions and 2-epoch B-band (equivalent) photometry of these sources are presented. The photometry suggests that this catalog includes most of the RR Lyrae population of this portion of M31.Comment: 22 pages, 3 tables, 5 figures, accepted for publication in A

The Reduction of Flavins by Borohydride: 3,4-Dihydroflavin

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66296/1/j.1432-1033.1969.tb00621.x.pd

Oxygen-rich dust production in IC 10

We report the detection of oxygen-rich circumstellar envelopes in stars of the nearby (700 kpc) starburst galaxy IC 10. The star formation history and the chemical environment of this galaxy makes it an ideal target to observe dust production by high-mass stars in a low-metallicity environment. The goal of this study is to identify oxygen-rich stars in IC 10 and to constrain their nature between asymptotic giant branch stars (AGBs), red supergiants (RSGs), and other infrared bright sources. We examine the mass-loss rate of the stars and compare to results obtained for the Magellanic Clouds. Our objectives are to (1) assess whether RSGs can be significant dust producers in IC 10, and (2), solve the discrepancy between the star formation history of IC 10 and the relatively low number of RSGs detected in the optical. We search for silicate dust in emission by using the spectral map observed with the Infrared Spectrograph on board the Spitzer Space Telescope. The optical (UBVRI) and infrared (JHK, Spitzer/IRAC and Spitzer/MIPS) photometry is used to assert the membership of the stars to IC 10 and disentangle between AGBs and RSGs. Radiative models are used to infer mass-loss rates and stellar luminosities. The luminosity and colors of at least 9 silicate emission sources are consistent with stars within IC 10. Furthermore, the photometry of 2 of these sources is consistent with RSGs. We derive dust mass-loss rates similar to the values found in the Magellanic Clouds. Accounting for the sample completeness, RSGs are not important contributors to the dust mass budget in IC 10.Comment: Accepted for publication in A&

WIYN Open Cluster Study XI: WIYN 3.5m Deep Photometry of M35 (NGC 2168)

We present deep BVI observations of the core of M35 and a nearby comparison field obtained at the WIYN 3.5m telescope under excellent seeing. These observations display the lower main sequence in BV and VI CMDs down to V = 23.3 and 24.6, respectively. At these faint magnitudes background field stars are far more numerous than the cluster stars, yet by using a smoothing technique and CMD density distribution subtraction we recover the cluster fiducial main sequence and luminosity function to V = 24.6. We find the location of the main sequence in these CMDs to be consistent with earlier work on other open clusters, specifically NGC 188, NGC 2420, and NGC 2477. We compare these open cluster fiducial sequences to stellar models by Baraffe et al. (1998), Siess et al. (2000), Girardi et al. (2000), and Yi et al. (2001) and find that the models are too blue in both B-V and V-I for stars below ~0.4 Mo. M35 contains stars to the limit of the extracted main sequence, at M ~ 0.10-0.15 Mo, suggesting that M35 may harbor a large number of brown dwarfs, which should be easy targets for near-IR instrumentation on 8-10m telescopes. We also identify a new candidate white dwarf in M35 at V = 21.36 +- 0.01. Depending on which WD models are used to interpret this cluster candidate, it is either a very high mass WD (1.05 +- 0.05 Mo) somewhat older (0.19-0.26 Gyr, 3-4 sigma) than our best isochrone age (150 Myr), or it is a modestly massive WD (0.67-0.78 Mo) much too old (0.42-0.83 Gyr) to belong to the cluster.Comment: 28 pages + 24 figures; to be published in the Sept, 2002 A

Tenfold Magnetoconductance in a Non-Magnetic Metal Film

We present magnetoconductance (MC) measurements of homogeneously disordered Be films whose zero field sheet conductance (G) is described by the Efros-Shklovskii hopping law $G(T)=(2e^2/h)\exp{-(T_o/T)^{1/2}}$. The low field MC of the films is negative with G decreasing 200% below 1 T. In contrast the MC above 1 T is strongly positive. At 8 T, G increases 1000% in perpendicular field and 500% in parallel field. In the simpler parallel case, we observe {\em field enhanced} variable range hopping characterized by an attenuation of $T_o$ via the Zeeman interaction.Comment: 9 pages including 5 figure

Polycyclic aromatic hydrocarbons in the dwarf galaxy IC 10

Infrared observations from the Spitzer Space Telescope archive are used to study the dust component of the interstellar medium in the IC~10 irregular galaxy. Dust distribution in the galaxy is compared to the distributions of H$\alpha$ and [SII] emission, neutral hydrogen and CO clouds, and ionizing radiation sources. The distribution of polycyclic aromatic hydrocarbons (PAH) in the galaxy is shown to be highly non-uniform with the mass fraction of these particles in the total dust mass reaching 4%. PAHs tend to avoid bright HII regions and correlate well with atomic and molecular gas. This pattern suggests that PAHs form in the dense interstellar gas. We propose that the significant decrease of the PAH abundance at low metallicity is observed not only globally (at the level of entire galaxies), but also locally (at least, at the level of individual HII regions). We compare the distribution of the PAH mass fraction to the distribution of high-velocity features, that we have detected earlier in wings of H$\alpha$ and SII lines, over the entire available galaxy area. No conclusive evidence for shock destruction of PAHs in the IC~10 galaxy could be found.Comment: Accepted for publication in Astronomy Report

Massive runaway stars in the Large Magellanic Cloud

The origin of massive field stars in the Large Magellanic Cloud (LMC) has long been an enigma. The recent measurements of large offsets (~100 km/s) between the heliocentric radial velocities of some very massive (O2-type) field stars and the systemic LMC velocity provides a possible explanation of this enigma and suggests that the field stars are runaway stars ejected from their birth places at the very beginning of their parent cluster's dynamical evolution. A straightforward way to prove this explanation is to measure the proper motions of the field stars and to show that they are moving away from one of the nearby star clusters or OB associations. This approach however is complicated by the large distance to the LMC, which makes accurate proper motion measurements difficult. We use an alternative approach for solving the problem, based on the search for bow shocks produced by runaway stars. The geometry of detected bow shocks would allow us to infer the direction of stellar motion and thereby to determine their possible parent clusters. In this paper we present the results of a search for bow shocks around six massive field stars which were suggested in the literature as candidate runaway stars. Using archival (Spitzer Space Telescope) data, we found a bow shock associated with one of our program stars, the O2 V((f*)) star BI 237, which is the first-ever detection of bow shocks in the LMC. Orientation of the bow shock suggests that BI 237 was ejected from the OB association LH 82 (located at ~120 pc in projection from the star). A by-product of our search is the detection of bow shocks generated by four OB stars in the field of the LMC and an arc-like structure attached to the candidate luminous blue variable R81 (HD 269128). The geometry of two of these bow shocks is consistent with the possibility that their associated stars were ejected from the 30 Doradus star forming complex.Comment: 5 pages, 7 figures, accepted for publication in A&